The present invention relates to a coil sensitivity map generating method, parallel imaging method, and MRI (magnetic resonance imaging) apparatus, and more particularly to a coil sensitivity map generating method, parallel imaging method, and MRI apparatus that can improve homogeneity of an image obtained in a self-calibration by a parallel imaging method generally referred to as SENSE (SENSitivity Encoding).
Conventional techniques relating to SENSE are described in, for example, the following literature:    (1) Pruessmann, K. P., Weiger, M., Scheidegger, M. B., and Boesiger P., Magn. Reson. Med., 42, 952 (1999); and    (2) P. B. Roemer, W. A. Edelstein, C. E. Hayes, S. P. Souza, and O. M. Mueller, Magn. Reson. Med., 16, 192–225 (1990).
FIG. 5 is a flow chart showing an operation of parallel imaging processing in a conventional MRI apparatus.
At Step J1, a calibration scan is conducted for an entire FOV (field of view) using phased array coils Coil_n (n=1–N, N≧2) to obtain respective complex images Cn for the phased array coils Coil_n.
At Step J2, a square image Cn2 of each complex image Cn is produced, and an additive square root image As=sqrt{ΣCn2} of the square images Cn2 is produced. “Sqrt{ }” is a function for obtaining a square root.
At Step J3, a sensitivity factor Sn(p)′ at a pixel p in a phased array coil Coil_n is calculated by the following equation:Sn(p)′=Cn(p)/As(p),where a pixel value of a pixel p in the complex image Cn is represented as Cn(p), and a pixel value of a pixel p in the additive square root image As is represented as As(p).
At Step J4, sensitivity map Sn′ are generated based on the sensitivity factors Sn(p)′.
At Step J5, a scan is conducted with a smaller FOV using the phased array coils Coil_1–Coil_N to obtain respective complex images A1–AN of the phased array coils Coil_1–Coil_N.
At Step J6, a complex image for the entire FOV is produced from the sensitivity maps Sn′ and the complex images A1–AN.
At Step J7, Steps J5 and J6 are repeated for a desired number of images.
Since the conventional parallel imaging processing shown in FIG. 5 generates the sensitivity maps Sn′ using only the phased array coils Coil_n (“self-calibration”), there is no need to obtain a separate calibration image using a body coil, thus offering an advantage of a simplified operation.
However, while the sensitivity factors Sn(p)′ calculated using the additive square root image As=sqrt{ΣCn2} of the square images Cn2 of the complex images Cn offer an excellent S/N ratio, they are not optimal in view of image homogeneity. That is, the image obtained by the conventional self-calibration suffers from a disadvantage of large inhomogeneity.